A phenomenon called "repetitive runout" is known in the field of disk drives, and occurs if the axis of rotation of the disk is shifted from the center of the concentric data storage tracks. The shifting of the center of the tracks from the axis of rotation of the disk comes about because the disk spindle is usually of a different material than that of the disk. Accordingly, a large thermal excursion, together with a difference in the thermal expansion or contraction rate of the different materials, can cause shifting of the spindle relative to the balance of the disk.
As a result of the shift between the axis of rotation and the center of the tracks, the track being followed by the read/write magnetic head is displaced laterally in a sinusoidal fashion relative to the head as the disk rotates. This disturbance is what is referred to as repetitive runout.
Although the conventional track following servo system attempts to compensate for repetitive runout, and succeeds in doing so to some extent, not all of the repetitive runout disturbance can be compensated for by conventional servo track following systems, so that the accuracy of track following remains adversely affected.
It is known to prevent the shift of the axis of rotation of the disk by taking certain steps during the manufacturing process, such as thermal treatment of the disk drive prior to defining the tracks on the disk. However, these steps add complexity and cost to the manufacturing process.
It is also known to provide compensation data to be used in positioning the magnetic head. According to a conventional technique, this data is obtained by performing a spectral analysis of track following signals, using a fast Fourier transform (FFT), in order to obtain the magnitude and phase of the sinusoidal repetitive runout disturbance. However, other disturbances in the operation of the disk drive cause the phase and amplitude data obtained from the FFT to be somewhat inaccurate, so that repetitive runout compensation is not performed with a desired degree of accuracy.